Development and evolution of avian racket plumes: Fine structure and serial homology of the wire

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Abstract
Avian racket plumes, characterized by a vaned paddle-shaped tip borne on a wirelike segment, are among the most spectacular pennaceous contour feathers. This is the first detailed description of the fine structure of the wirelike segment in several racket plumes that acquire their shape exclusively during development, for the parrot Prioniturus discurus, the hummingbirds Ocreatus underwoodii and Loddigesia mirabilis, the kingfisher Tanysiptera carolinae, and the bird of paradise Parotia carolae. Scanning electron microscopy reveals that the wirelike segments are comprised of small, structurally simplified barbs located along a central rachis, indicating that racket plumes retain the basic structural organization of pennaceous contour feathers. The diverse structures provide evidence of several alterations to normal developmental processes, including arrested and disrupted development of the barbs. Many of the major differences among the five species in the fine structure of the wire apparently result from differences in the morphogenesis of barb ridges on the epidermal collar of the developing plume. The structure and morphogenesis of the racket plumes described herein are distinct from those described in motmots (Momotidae), indicating that racket plumes can evolve through convergence.